TRANSIT CONFIRMATION AND IMPROVED STELLAR AND PLANET PARAMETERS FOR THE SUPER-EARTH HD 97658 b AND ITS HOST

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Citation Van Grootel, V., M. Gillon, D. Valencia, N. Madhusudhan, D. Dragomir, A. R. Howe, A. S. Burrows, et al. “TRANSIT CONFIRMATION AND IMPROVED STELLAR AND PLANET PARAMETERS FOR THE SUPER-EARTH HD 97658 b AND ITS HOST STAR.” The Astrophysical Journal 786, no. 1 (April 8, 2014): 2. © 2014 The American Astronomical Society

As Published http://dx.doi.org/10.1088/0004-637X/786/1/2

Publisher IOP Publishing

Version Final published version

Citable link http://hdl.handle.net/1721.1/94587

Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The Astrophysical Journal, 786:2 (11pp), 2014 May 1 doi:10.1088/0004-637X/786/1/2 C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

TRANSIT CONFIRMATION AND IMPROVED STELLAR AND PLANET PARAMETERS FOR THE SUPER-EARTH HD 97658 b AND ITS HOST STAR

V. Van Grootel1,10, M. Gillon1, D. Valencia2, N. Madhusudhan3, D. Dragomir4,5,A.R.Howe6, A. S. Burrows6, B.-O. Demory3,7,D.Deming8, D. Ehrenreich9, C. Lovis9, M. Mayor9, F. Pepe9, D. Queloz3,9,R.Scuflaire1, S. Seager7, D. Segransan9, and S. Udry9 1 Institut d’Astrophysique et de Geophysique,´ UniversitedeLi´ ege,` 17 Alleedu6Ao´ ut,ˆ B-4000 Liege,` Belgium; [email protected] 2 Department of Physical and Environmental Sciences, University of Toronto, 1265 Military Trail, Toronto, ON, M1C 1A4, Canada 3 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK 4 Las Cumbres Observatory Global Telescope Network, 6740 Cortona Dr. Suite 102, Goleta, CA 93117, USA 5 Department of Physics, Broida Hall, UC Santa Barbara, CA 93106, USA 6 Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA 7 Department of Earth, Atmospheric and Planetary Sciences, Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA 8 Department of Astronomy, University of Maryland, College Park, MD 20742-2421, USA 9 Observatoire de Geneve,` UniversitedeGen´ eve,` 51 Chemin des Maillettes, CH-1290 Sauverny, Switzerland Received 2014 January 2; accepted 2014 February 25; published 2014 April 8

ABSTRACT Super-Earths transiting nearby bright are key objects that simultaneously allow for accurate measurements of both their and radius, providing essential constraints on their internal composition. We present here the confirmation, based on Spitzer transit observations, that the super-Earth HD 97658 b transits its host star. HD 97658 is a low-mass (M∗ = 0.77 ± 0.05 M) K1 dwarf, as determined from the Hipparcos parallax and modeling. To constrain the planet parameters, we carry out Bayesian global analyses of Keck–High Resolution Echelle Spectrometer (Keck-HIRES) radial velocities and Microvariability and Oscillations of STars (MOST) and = +0.83 = +0.098 Spitzer photometry. HD 97658 b is a massive (MP 7.55−0.79 M⊕) and large (RP 2.247−0.095R⊕ at 4.5 μm) super-Earth. We investigate the possible internal compositions for HD 97658 b. Our results indicate a large rocky component, of at least 60% by mass, and very little H–He components, at most 2% by mass. We also discuss how future asteroseismic observations can improve the knowledge of the HD 97658 system, in particular by constraining its age. Orbiting a bright host star, HD 97658 b will be a key target for upcoming space missions such as the Transiting Exoplanet Survey Satellite (TESS), the Characterizing Exoplanet Satellite (CHEOPS), the Planetary Transits and Oscillations of stars (PLATO), and the James Webb Space Telescope to characterize thoroughly its structure and atmosphere. Key words: binaries: eclipsing – planetary systems – stars: individual (HD 97658) – techniques: photometric – techniques: radial velocities Online-only material: color figures, machine-readable table

1. INTRODUCTION Absil et al. 2013; Kuzuhara et al. 2013). Transiting super-Earths around bright nearby stars are therefore essential for shedding Among the diversity of the exoplanetary population, the class light on their true nature and origin. of the so-called “super-Earths,” with a mass of a few Earth HD 97658 is the second-brightest (V = 7.7, K = 5.7) , is of utmost interest. First, they do not exist in the solar host star found to be transited by a super-Earth (the bright- system, leaving us with only extrasolar planets for detailed est being 55 Cnc; Winn et al. 2011; Demory et al. 2011). studies. Second, they seem to be extremely common in the HD 97658 b was detected from radial velocity (RV) mea- Galaxy (Fressin et al. 2013; Petigura et al. 2013;Howard2013). surements with Keck–High Resolution Echelle Spectrometer Third, a wide variety of interiors are theoretically possible (Keck-HIRES) spectroscopy, with a minimum mass MP sin i = for a small range of radii (1

1 The Astrophysical Journal, 786:2 (11pp), 2014 May 1 Van Grootel et al. planets (Gillon et al. 2011). HD 97658 b was observed in 2013 August with the Spitzer Infrared Array Camera (IRAC; Fazio et al. 2004)at4.5μm. Spitzer already proved to be the tool of choice to perform exquisitely precise and continuous photom- etry for the detection and characterization of the ultrashallow transits of super-Earths such as HD 40307 b (for which transits are firmly discarded; Gillon et al. 2010) and 55 Cnc e (Demory et al. 2011). We present here the confirmation of the transit- ing nature of HD 97658 b from Spitzer observations. We also present global Bayesian analyses of Keck–HIRES RV data and MOST and Spitzer photometry in order to derive as accurately as possible the parameters of HD 97658 b. Ultimately, the precision for planetary parameters is related to the precision achieved for the stellar parameters because plane- tary masses and radii as determined by transit and RV measure- ments cannot be determined independently from the properties of their host stars. Atmospheric stellar parameters such as ef- fective temperature Teff and are determined from spectroscopy. The stellar radius, at least for nearby stars, can be derived from the (from parallax measurements) and the spectroscopic using the classical equa- = 2 4 tion of stellar physics L∗ 4πR∗σTeff. Obtaining the is a more delicate process that can be achieved by a stellar Figure 1. Raw HD 97658 Spitzer IRAC light curve (blue dots), with the global evolution code using T , metallicity, and luminosity as inputs. photometric model overimposed (red curve) made of the photometric eclipse eff model of Mandel & Agol (2002) multiplied by the baseline model representing The stellar age is usually poorly constrained in this process the Spitzer instrumental effects; see text for details. (Soderblom 2010). A powerful method for providing accurate (A color version of this figure is available in the online journal.) and precise stellar radius, mass, and age is asteroseismology, by modeling the oscillation spectrum of the star. The Convection, We first converted fluxes from the Spitzer units of spe- Rotation, and planetary Transits (CoRoT; Baglin et al. 2006) cific intensity (MJy sr−1) to photon counts; then aperture and Kepler (Koch et al. 2010) space missions were based on photometry was performed on each subarray image with the this complementarity, whereas the future Transiting Exoplanet IRAF/DAOPHOT13 software (Stetson 1987). We tested different Survey Satellite (TESS; Ricker et al. 2010) and Planetary Tran- aperture radii and background annuli, the best result (the one sits and Oscillations of stars (PLATO; Rauer et al. 2014) space that gives the lowest white noise, by minimizing the rms of the missions will also include significant asteroseismic programs. residuals, and the lowest red noise, by following the approach We also investigate here how future asteroseismic observations presented in Gillon et al. 2006) being obtained with an aperture of HD 97658 can help to further improve the knowledge of the radius of 3 pixels and a background annulus extending from 11 star and planet parameters. to 15.5 pixels from the point-spread function (PSF) center. The This paper is organized as follows. The Spitzer observations PSF center was measured by fitting a two-dimensional Gaussian and their reduction are presented in Section 2, while host profile on each image. The x–y distribution of the measurements star modeling is presented in Section 3. Section 4 is devoted was examined, and we discarded the few measurements having to the analysis of the Spitzer data, while Section 5 presents a very different position than the bulk of the data. For each block global Bayesian analyses of RV, MOST, and Spitzer data. We of 64 subarray images, we discarded the discrepant values for discuss the results and provide avenues to further improve the the measurements of flux, background, x, and y positions using a characterization of HD 97658 b and its host star in Section 6. σ median clipping (5σ for the flux and 10σ for the other param- Conclusions and prospects are given in Section 7. eters), and the resulting values were averaged, the photometric error being taken as the error on the average flux measurement. 2. SPITZER PHOTOMETRY A20σ running median clipping was used on the resulting light curve to discard totally discrepant fluxes (due, e.g., to cosmic We monitored HD 97658 with Spitzer’s IRAC camera on 2013 rays). In the end, only 0.05% of the measurements were re- August 10 from 13:01:00 to 18:27:00 UT, corresponding to a jected. The blue dots of Figure 1 represent the resulting IRAC transit window11 as computed from the MOST transit ephemeris photometric light curve of HD 97658. These data, which are (Dragomir et al. 2013). These Spitzer data were acquired in the directly used in our MCMC algorithm (see Sections 4 and 5), context of the Cycle 9 program 90072 (PI: M. Gillon) dedicated can be found on Table 1 (the full version is available online in to the search for the transits of RV-detected low-mass planets. the form of a machine-ready table). They consist of 2320 sets of 64 individual subarray images obtained at 4.5 μm with an integration time of 0.08 s. They 3. HOST STAR PARAMETERS are available on the Spitzer Heritage Archive database12 under the form of 2320 Basic Calibrated Data files calibrated by the An accurate knowledge of the host star, including the stellar standard Spitzer reduction pipeline (version S19.1.0). mass and radius, is needed in any exoplanetary modeling. In addition, the age of the star is an excellent proxy for the age

11 The “transit window” is the window of time within which the transit is 13 IRAF is distributed by the National Optical Astronomy Observatory, which likely to occur. is operated by the Association of Universities for Research in Astronomy, Inc., 12 http://sha.ipac.caltech.edu/applications/Spitzer/SHA under cooperative agreement with the National Science Foundation.

2 The Astrophysical Journal, 786:2 (11pp), 2014 May 1 Van Grootel et al.

Table 1 Spitzer Photometric Time Series of HD 97658 as Used by Our MCMC Algorithm

BJD_UTC Flux Error dX dY FWHM FWHM_X FWHM_Y sky () (pixel) (pixel) (pixel) (pixel) pix e− 6523.0421407213 1.00042472 6.56049341e-04 15.99832813 16.06571875 1.1364 1.28568133 1.32566666 3.624313 6523.0422367211 1.00001116 6.42787543e-04 16.01006250 16.06017187 1.1312 1.27501281 1.31487295 2.482975 6523.0423342212 0.99991533 5.79726271e-04 16.00939063 16.05585937 1.1294 1.27034785 1.31009623 1.657124 6523.0424312213 1.00122759 6.76242624e-04 16.00635937 16.06600000 1.1361 1.28203944 1.32744292 4.489830 6523.0425283214 0.99966335 6.49723786e-04 15.98585937 16.07160937 1.1443 1.30291407 1.33813047 3.095959

(This table is available in its entirety in a machine-readable form in the online journal. A portion is shown here for guidance regarding its form and content.) of its planets because they are expected to have formed within a few million of each other. Two sources are available for the atmospheric stellar parameters of HD 97658 (Howard et al. 2011; Henry et al. 2011). From Howard et al. (2011, −0.4 respectively, Henry et al. 2011), the effective temperature is Teff = 5170 ± 44 K (respectively, Teff = 5119 ± 44 K), the sur- 13.7 Gyr = ± ± 4.1 Gyr face gravity log g 4.63 0.06 (respectively, 4.52 0.06), and 10.0 Gyr [Fe/H] =−0.23 ± 0.03 (respectively, −0.30 ± 0.03). We as- ) sumed here that [Fe/H] represents the global metallicity with /L −0.45 respect to the , defined as [log(Z/X)∗ −log(Z/X)], where ∗ X and Z are the fractional mass of hydrogen and elements heavier L than helium, respectively. A Hipparcos parallax measurement log( 7.2 Gyr 1.0 Gyr of HD 97658 is available: π = 47.36 ± 0.75 mas (van Leeuwen 11.0 Gyr 2007), giving a distance of 21.11 ± 0.34 pc. Using observed − . magnitudes of Koen et al. (2010) and bolometric corrections of 0 5 Flower (1996) (with Mbol, = 4.73; Torres 2010), this trans- = ± M∗ =0.73M ,Fe/H=−0.26, Yarb lates to a stellar luminosity of L∗/L 0.355 0.018. As is M . M − Y standard, all of the errors cited in this section and throughout ∗ =077 ,Fe/H= 0.23, G M∗ =0.81M ,Fe/H=−0.20, Y the paper are in the 1σ range (68.3% probability). We used the effective temperature, metallicity, and luminosity 3.72 3.715 3.71 3.705 with their respective errors as inputs for stellar evolution log Teff (K) modeling with the Code Liegeois´ d’Evolution Stellaire (CLES) Figure 2. Evolutionary tracks in a log Teff − log(L∗/L) diagram for HD code (Scuflaire et al. 2008b). In all evolutionary computations 97658, for various masses and that respect the observational we used the mixing-length theory (MLT) of convection (Bohm-¨ constraints (L∗, Teff ,[Fe/H]). Several initial mixtures, in particular for the Vitense 1958) and the Coulomb Eggleton Faulkner Flanery helium abundance (Y, YG,andYarb) were also considered (see text for details). − (CEFF) equation of state (Christensen-Dalsgaard & Daeppen The age of the star when it crosses the 1σ box log Teff log(L∗/L) is also 1992). We considered here the most recent solar mixture14 of indicated. (A color version of this figure is available in the online journal.) Asplund et al. (2009), giving for the present Sun (Z/X) = 0.0181. We used for this metallic mixture the Opacity at Livermore (OPAL) opacity tables (Iglesias & Rogers 1996)for We computed many evolutionary tracks for several masses the high temperatures and the ones of Ferguson et al. (2005) with the constraints of having consistent metallicities and for the low temperatures. The surface boundary conditions effective temperatures as given by spectroscopy and having are given by atmospheres computed within the Eddington consistent with the one derived from the Hipparcos approximation. Microscopic diffusion (gravitational settling) parallax. No stellar models younger than the universe were is taken into account, but no radiative accelerations of metals found to have a luminosity L∗/L = 0.355 ± 0.018 within the were included given the low mass we expect for the host star 1σ range of effective temperature and metallicity given by Henry (Escobar et al. 2012). For the same reason, no convective core is et al. (2011), which are therefore discarded from stellar evolution expected, so no overshooting was considered here. Finally, the modeling. Within the 1σ range given by Howard et al. (2011) α parameter of the MLT was kept fixed to the solar calibration for the metallicity and effective temperature, the values of stellar (αMLT = 1.8). Because the helium atmospheric abundance mass are M∗ = 0.77 ± 0.05 M (Figure 2). Unfortunately, no cannot be directly measured from spectroscopy in low-mass useful constraints on the stellar age were obtained: such stars stars such as HD 97658, we computed evolutionary tracks with can reach L∗/L = 0.355 ± 0.018 when they are a few Gyr old three initial helium abundances: the solar value, a value labeled only if they are ∼ 0.82 M, but they can be much older, up to the YG that increases with Z (as expected if the local medium age of the universe, if they are less massive (see Figure 2). This follows the general trend observed for the chemical evolution also strongly depends on the initial mixture. All evolutionary of galaxies; Izotov & Thuan 2010), and an arbitrary value tracks that respect the observational constraints on Teff, L∗, and Yarb = 0.26 close to the most recent primordial value from [Fe/H] correspond to stars that are on the main sequence in the the big bang nucleosynthesis (Izotov & Thuan 2010). H-core burning phase. Combining the stellar luminosity L∗/L = 0.355 ± 0.018 14 “Mixture” is stellar physics jargon that means the proportions of X, Y,and with the effective temperature of Howard et al. (2011) results Z, with X + Y + Z = 1. in a stellar radius R∗/R = 0.74 ± 0.03. These values are

3 The Astrophysical Journal, 786:2 (11pp), 2014 May 1 Van Grootel et al. somewhat different from the ones cited by Howard et al. (2011) and also by Henry et al. 2011 (which were taken by Dragomir et al. 2013). The discrepancy comes from their announced luminosity L∗/L = 0.30 ± 0.02 and absolute magnitude MV = 6.27 ± 0.10, which are not consistent with the Hipparcos parallax (van Leeuwen 2007; see also Table 1 of Koen et al. 2010, HD 97658 ≡ HIP 54906).

4. SPITZER CONFIRMS THE TRANSITING NATURE OF HD 97658 b The Spitzer light curve was analyzed with the adaptative MCMC algorithm presented in Gillon et al. (2012b, and ref- erences therein), with the aim to confirm or refute the transiting nature of HD 97658 b and the ephemeris of Dragomir et al. (2013). The MCMC algorithm is a Bayesian inference method based on stochastic simulations that sample the posterior prob- ability distributions of adjusted parameters for a given model. Our MCMC implementation uses the Metropolis–Hasting algo- rithm (see, e.g., Carlin & Louis 2008) to perform this sampling. Our nominal model was based on a star and a transiting planet on a Keplerian orbit about their center of mass. We modeled the eclipse photometry with the photometric eclipse model of Mandel & Agol (2002), multiplied by a baseline model rep- Figure 3. HD 97658 Spitzer IRAC light curve binned to intervals of 5 minutes, resenting the low-frequency instrumental effects. This baseline with the best-fit transit model overimposed. The vertical solid line is the propagated midtransit time of Dragomir et al. (2013), with its 1σ errors (dashed model was a sum of a second-order polynomial in the x and y vertical lines). positions of the PSF center, first-order polynomials in the width (A color version of this figure is available in the online journal.) of the PSF in the x and y directions, and a first-order polyno- mial of the logarithm of time. The two first kinds of polynomial aimed to model the “pixel-phase effect” affecting the IRAC stellar parameters with their Gaussian priors in the MCMC sim- InSb arrays, whereas the last one represented the sharp increase ulation to properly propagate the errors and to accurately derive of the Spitzer detectors’ response, generally called “the ramp” the physical parameters from the jump parameters. If a jump pa- in the literature (see, e.g., Knutson et al. 2008). This baseline rameter is not constrained by observational data, its a posteriori model was elected based on the minimization of the Bayesian distribution will be the same as its a priori distribution, but we information criterion (Schwarz 1978). The final global photo- can use the resulting distribution to propagate errors correctly. metric model (eclipse model multiplied by the baseline model) The good convergence and the quality of the sampling of is shown by the red curve in Figure 1, which nicely illustrates the the MCMC simulation were successfully checked using the importance of detrending operations on Spitzer raw photometry statistical test of Gelman & Rubin (1992), by verifying that in order to extract the scientific information. Finally, quadratic the so-called potential reduction factors for all jump parameters limb-darkening was assumed, with values deduced from the are close to unity (within 1%). Figure 3 shows the Spitzer tables of Claret & Bloemen (2011) for the appropriate Spitzer IRAC photometry corrected for the systematics and binned filters and stellar parameters. into 5 minute wide intervals with the best-fitting eclipse model The jump parameters, i.e., the parameters over which the superimposed. The vertical lines show the propagated (17 MCMC random walk (here made of 10 chains of 104 steps) planetary orbits later) MOST midtransit time, with 1σ errors, occurs, were the transit depth dF, the transit width (from first of Dragomir et al. (2013). The Spitzer-detected transit at = +0.00062 to last contact) W, the transit timing (time of minimum light) T0 2456523.12544−0.00059 (BJD_TDB) thus fully confirms,  T0, and the impact parameter b = a cos i/R∗. We assumed a within 1σ , the ephemeris provided by Dragomir et al. (2013). uniform prior distribution for all of these jump parameters. The P was also a jump parameter but this time with a 5. GLOBAL BAYESIAN ANALYSES Gaussian prior based on the value from Dragomir et al. (2013). In order to get the strongest constraints on the system In our MCMC implementation, it means that we imposed a parameters, we performed several global Bayesian analyses Bayesian penalty: using as input data not only our Spitzer transit photometry but 2 also the detrended MOST transit light curves (Dragomir et al. (P − PD13) BPephemeris = , (1) 2013) and the 171 published Keck–HIRES RVs acquired and σ 2 PD13 reduced using the same techniques as in Howard et al. (2011) = = and compiled by Dragomir et al. (2012, 2013). where PD13 9.4909 days and σPD13 0.0016 days (Dragomir et al. 2013). Gaussian priors were also imposed on the stel- 5.1. Keck–HIRES RV and Spitzer Photometry Data lar mass, luminosity, effective temperature, and metallicity (see Section 3), i.e., we also applied a Bayesian penalty similar to We performed an MCMC simulation made of 10 chains of Equation (1) for these parameters. The merit function used in our 5 × 104 steps including HIRES RV data and Spitzer photom- MCMC simulation was therefore the sum of the χ 2 for each data etry. We used a classical Keplerian model for the RVs (no set (quadratic sum of the difference between the model and the Rossiter–McLaughlin effect was detected). The jump param- data) and of the Bayesian penalties. It is also important to include eters in our MCMC simulation were the transit depth dF,the

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Table 2 Median and 1σ Limits of the Posterior Distributions Derived for HD 97658 and Its Planet from Our MCMC Analysis of Spitzer Photometry and Keck–HIRES RVs

Parameter Symbol Value Unit Jump parameters Jump parameter, uniform prior Transit depth, Spitzer dF 773 ± 42 ppm Transit width W 0.1187 ± 0.0012 days +0.00060 Midtransit time-2450000 T0 6523.12540−0.00056 BJD_TDB  = +0.13 Impact parameter b a cos i/R∗ 0.35−0.21 R∗ Orbital period P 9.4903+0.0016 days √ −0.0015 e cos ω 0.05+0.18 √ −0.20 +0.13 e sin ω 0.18−0.23 +0.56 K2 5.76−0.58 Jump parameter, Gaussian prior Stellar effective temperature Teff 5170 ± 50 K Stellar metallicity [Fe/H] −0.23 ± 0.03 Stellar luminosity L∗ 0.355 ± 0.018 L Stellar mass M∗ 0.77 ± 0.05 M Derived stellar parameters +0.024 Stellar radius R∗ 0.741−0.023 R +0.23 Stellar density ρ∗ 1.89−0.20 ρ +0.047 Stellar log g∗ 4.583−0.054 +0.00079 Limb-darkening coeff. u1 0.07313−0.00078 Limb- darkening coeff. u2 0.1442 ± 0.0013 Distance d 21.11 ± 0.34 pc Derived planet parameters +0.00075 Radii ratio RP /R∗ 0.02780−0.00077 +0.098 Planet radius (at 4.5μm) RP 2.247−0.095 R⊕ +0.83 Planet mass MP 7.55−0.79 M⊕ +0.70 −3 Planet density ρP 3.90−0.61 gcm +0.059 Planet surface gravity log gP 3.166−0.061 +0.52 i 89.14−0.36 deg +0.0017 Orbital semi-major axis a 0.080−0.0018 AU +0.057 e 0.078−0.053 +65 Argument of the periastron ω 71−63 deg +0.26 −1 RV orbital semiamplitude K 2.73−0.27 ms +12 Planet equilibrium temperature Teq 757−13 K transit width (from first to last contact) W, the transit timing (time Table 2 shows the median values and 68.3% probability  of minimum light) T0, the impact parameter√ b = a cos√ i/R∗, interval for the jump parameters mentioned above given by the orbital period P, the two parameters e cos ω and e sin ω our MCMC simulation of Spitzer photometry and Keck–HIRES (e is the eccentricity and ω the argument of periastron), and the RVs as well as the derived stellar and planetary physical K2 parameter√ (related to the RV orbital semiamplitude K via parameters. The good convergence and the quality of the 2 1/3 K2 = K 1 − e P ). We also allowed the quadratic limb- sampling of the 10 chains of the MCMC simulation were darkening coefficients u1 and u2 to float in this MCMC simula- successfully checked using again the Gelman & Rubin (1992) tion, using as jump parameters not these coefficients themselves statistical test (all jump parameters have potential reduction but the combinations c1 = 2×u1+u2 and c2 = u1–2×u2 to mini- factors near unity within 1%). mize the correlation of the obtained uncertainties (Holman et al. 2006). The prior distributions of these limb-darkening coeffi- 5.2. Keck–HIRES RV, MOST, and Spitzer Data cients directly depend on the ones on the effective temperature Finally, we repeated the global Bayesian analysis, but this and metallicity, for which Gaussian priors, i.e., Bayesian penal- time including the three continuous MOST transits. We only = ± ties similar to Equation (1), were imposed (Teff 5170 50 K used these three light curves acquired when the star was fully in and [Fe/H] =−0.23 ± 0.03). Gaussian priors were also im- the continuous viewing zone of the satellite, the two other light posed on the stellar mass (M∗ = 0.77 ± 0.05 M) and luminos- curves showing a much poorer quality due to gaps in coverage ity (L∗ = 0.355 ± 0.018 L). Again, it is important to include and higher levels of correlated noise. As discussed in Dragomir these parameters with their Gaussian priors in the MCMC simu- et al. (2013), the MOST reduction pipeline suppresses the depth lation in order to correctly propagate the errors and to accurately of a transit signal, by an amount of ∼10%, as indicated by transit derive the physical parameters from the jump parameters. injection tests in these three MOST transits. To include this effect

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40000 cf=0.5 hh=0.25 HP-26b GJ3470b K18-c 30000 HP-11b K11e U N K30b K4-b cf=0 hh=0 K36-c GJ 436b K11d K20c 20000 K11c cf=0.5 hh=0 GJ 1214b K11f cf=0.9 hh=0.01 HD 97658B K68-b 55 Cnc-e K18-b K20-b K11-b K36-b 500 K < Teq < 700 K 10000 C7-b 700 K < T < 900 K Radius [kms] eq K10-b 900 K < Teq < 1100 K K78-b 1100 K < T < 1300 K 8000 eq 1300 K < Teq < 1500 K 1500 K < Teq < 1700 K 6000 E 1700 K < Teq < 1900 K 1900 K < Teq < 2100 K 5000 2100 K < Teq < 2300 K 0.4 0.7 123571020

Mass [MEarth] Figure 4. Planetary mass–radius diagram of HD 97658 b and other low-mass exoplanets. The planets are color coded according to their equilibrium temperature. The orange line is the mass-radius relationship for a pure silicate planet (the lightest rocky composition), the green is an Earth-like composition (2/3 silicate mantle with 10% iron by mol, 1/3 iron core), and the brown line is an iron-rich composition (37% silicate mantle, 63% iron core). The blue lines show specific mass-radius relations. “Cf” stands for core fraction, and “hh” is for bulk hydrogen and helium by mass: (1) core fraction = 0, H–He = 0, and water/ices = 1; (2) core fraction = 0.5, H–He = 0 and water/ices = 0.5; (3) core fraction = 0.5, H–He = 0.25 and water/ices = 0.25; (4) core fraction = 0.9, H–He = 0.01 and water/ices = 0.09. (A color version of this figure is available in the online journal.) and the uncertainty on it, we assumed a dilution of 10% ± 2% 75 times more massive than Mars, it clearly has a much lighter for the MOST photometry. We carried out a global MCMC composition. analysis with the same number of steps and chains, the same In Figure 4 we show how HD 97658 b compares to the jump parameters, the same Gaussian priors, and the same check other detected low-mass planets for which we have measured of the convergence and sampling quality as in Section 5.1.The mass and radius. It is worth noting that the mass and radius of most interesting result is the transit depth in visible, from MOST HD 97658 b are very similar to Kepler 68 b (Gilliland et al. = +81 photometry: dFMOST 949−75 ppm. This translates to a planet 2013). To illustrate that HD 97658 b has both a low H–He = +0.14 content and a high core-mass fraction, within the limitations of radius RP 2.49−0.13 R⊕, larger at the 2σ level than the planet radius at 4.5 μm derived from Spitzer photometry (Table 2). a mass–radius diagram, we show mass–radius relations (blue = This may be related to some instrumental MOST systematics lines) for four specific compositions, calculated at Teq 700 K: not fully corrected beyond the 2% uncertainty we took here on 1. a water planet (no core and no H–He) (dotted blue line); the dilution effect. This could also be related to the atmospheric 2. core fraction of 50% and no H–He (dash-dotted line); composition of HD 97658 b, as we discuss in the next section. 3. core fraction of 50% and H–He = 25% (long dashed line); We note that our planet radius based on MOST photometry is 4. core fraction of 90% and H–He = 1% (solid line). slightly higher than the one found by Dragomir et al. (2013) +0.18 A water-only composition (1) sits above the data for (2.34−0.15 R⊕), although it is within their 1σ uncertainties. This is directly related to the more luminous and, therefore, larger HD 97658 b, even though it has no H–He. If we consider larger radius star we modeled (see Section 3). core-mass fractions, only those above 50% ((2)–(4)) start to approach the data for HD 97658 b. If we include any amount 6. DISCUSSION of H–He the amount of core needs to be larger. One possible composition for HD 97658 b is a planet with an Earth-like core 6.1. HD 97658 b, A Key Object for Super-Earth of 90%, 1% H–He, and 9% water/ices (Figure 4). Characterization It is not possible to show all the possible compositions for 6.1.1. Internal Composition of HD 97658 b a low-mass planet using a mass–radius diagram; for this we resort to a ternary diagram (Figure 5). We computed various Our MCMC results (see Table 2) give a planetary radius theoretical internal structures using the internal structure model = +0.098 of RP 2.247−0.095 R⊕ as measured in the IRAC 4.5 μm described in Valencia et al. (2013), suitable for rocky and = +0.83 channel and a planetary mass of MP 7.55−0.79 M⊕.This gaseous planets. Ternary diagrams relate the composition in yields a super-Earth with an intermediate average density terms of Earth-like nucleus fraction, water+ices fraction, and = +0.70 −3 (ρP 3.90−0.61 gcm ), close to the average density of H–He fraction to total mass, to the radius for a specific planetary −3 Mars (ρ♂ = 3.9335 g cm ). Valencia et al. (2006) proposed mass. Each vertex corresponds to 100%, and the opposite side a model for the mass–radius relationship for rocky planets as to 0% of a particular component, by mass. The color bar shows = 4 MP aRP , where a depends on the composition. For the the radius in terms of Earth radii, and the gray lines are the same composition, the average density of a planet therefore isoradius curves labeled in terms of Earth radii. The possible ∼ 3 ∼ 0.25 increases as ρP MP /RP MP . Given that HD 97658 b is compositions for HD 97658 b are in the lower right corner of

6 The Astrophysical Journal, 786:2 (11pp), 2014 May 1 Van Grootel et al.

0% 100% 40

10% 90% HD 97658b 25 H-He 20% 80% M=7.55 ME 30% 16 R

70% 25 p

l

a n

20 40% e t

60% 10 /R E

15 50% a r

50% t 12 6.3 h 60% 40% 10 4.0 8 70% 30% Earth-like

6 80% Nucleus 20% 2.5 5

90% 10% 4 3 2.5 100% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0%

H2O

Figure 5. Ternary diagram for HD 97658 b. This diagram shows the radius for all possible compositions for a planet of a specific mass (in this case 7.55 M⊕). Each point depicts a unique planetary composition from a combination of H–He, H2O + ices, and rocky Earth-like nucleus (33% iron core below a 67% silicate mantle). Each vertex corresponds to 100% of each compositional end member (with H–He on top, H2O + ices in the lower left corner, and Earth-like nucleus on the lower right corner), with 0% on the opposite line. The color bar shows the radius in terms of Earth radii, and the gray lines are the isoradius curves labeled in terms of Earth radii. The black shaded region in the lower right corner shows the possible compositions for HD 97658 b, and the width of the shaded region takes into account the uncertainty in the mass and radius (for all combinations between M + ΔM, R − ΔR,andM − ΔM, R + ΔR, with ΔM and ΔR the 1σ error in mass and radius). This compact planet has at least 60% Earth-like nucleus by mass and between 0%–40% bulk water+ices content. (A color version of this figure is available in the online journal.)

Figure 5 (shaded in black) and correspond to a bulk composition of terrestrial planets. Detailed transmission spectroscopy has of H–He of less than 2%, water+ices 0%–40%, and rocks in been reported for only one super-Earth to date, GJ 1214 b, for excess of 60%. The maximum H–He is obtained for a rock which the observations indicate a flat spectrum over a wide fraction of 92%–95% and the rest water+ices. If the planet had wavelength range of ∼0.5–5 μm (e.g., Bean et al. 2010;Berta no water, the amount of H–He would be less than 8 × 10−3 et al. 2011;Desert´ et al. 2011;deMooijetal.2012). The by mass. If it had no H–He, the amount of water+ices is featureless spectrum of GJ 1214 b suggests the likely presence of 15%–40% by mass (the range reflects the 1σ errors in mass and thick clouds in the atmosphere (Howe & Burrows 2012; Miller- radius). In comparison, GJ 1214 b admits at most 7% of H–He, Ricci Kempton et al. 2012; Benneke & Seager 2013;Morley and it could be made out of 100% water+ices (although very et al. 2013; Kreidberg et al. 2014), which might be obscuring unlikely). Furthermore, GJ 1214 b permits a very large range the spectral features of volatile species under the cloud cover, in rocky component (0%–97%), whereas HD 97658 b is more due to which its atmospheric composition is still unknown. compact and requires a large proportion of rocks (60%–99%), In this section, despite the only 2σ significance of the thus requiring a formation mechanism that captures enough planetary radius discrepancy between MOST and Spitzer data, solid material. Unfortunately, as discussed in Valencia et al. and the possibility of instrumental MOST systematics not fully (2013), internal structure models do not constrain the mixing corrected (see Section 5.2), we attempted to interpret the ratio between H–He and water+ices in the atmosphere. Thus, origin of the discrepancy based on atmospheric modeling of they are limited in guiding spectroscopic studies, which we HD 97658 b. We also assumed here that the stellar variability discuss in the next section. of HD 97658 is weak enough to infer atmospheric constraints directly from the transit-depth measurements. With a projected 6.1.2. Constraints on Atmospheric Composition rotational velocity of 0.5 ± 0.5 km s−1, a magnetic cycle at ± Transmission spectra of super-Earths can provide constraints least 6 yr long, and a of 38.5 1 days (Henry on the atmospheric composition of the day–night terminator et al. 2011), HD 97658 is most probably a very quiet star. region of the planetary atmosphere, in particular on the mean Furthermore, occulted star-spots leave a clear structure in the molecular weight (MMW) of the atmosphere (e.g., Miller-Ricci transit light curve, which we do not observe with MOST or with et al. 2009) or on the presence of clouds. The atmospheric Spitzer. compositions of super-Earths are a subject of active debate at The transit-depth measurements in the two bandpasses, the present because it is unknown if their atmospheres are H/He- MOST bandpass in the visible centered at 0.525 μm and rich like those of ice giants in the solar system or if they are the Spitzer IRAC bandpass in the infrared at 4.5 μm, place complementary constraints on the atmospheric composition. rich in heavier molecules such as H2O, CO, or N2, like those

7 The Astrophysical Journal, 786:2 (11pp), 2014 May 1 Van Grootel et al.

Figure 6. Model transit spectra of super-Earth HD 97658 b with cloud- Figure 7. Model transit spectra of super-Earth HD 97658b with atmospheres free atmospheres. The black circles with error bars show the two observed consisting of tholin haze particles. The solid curves show spectra of three model photometric transit depths in the MOST and Spitzer bandpasses centered at atmospheres with tholin hazes of different particle sizes and densities based on 0.525 μm and 4.5 μm, respectively. The solid black curves at the bottom the models of Howe & Burrows (2012). All of the models contain monodispersed show the corresponding bandpasses. The Spitzer 3.6 μm bandpass is also tholin hazes placed in an H-rich solar-abundance atmosphere uniformly in the shown for comparison. The solid curves show four model spectra of cloud- 10−4–10−6 bar pressure range. The particle sizes and densities for the different free atmospheres with different chemical compositions, computed using the models are shown in the legend. All three haze models fit the data very well modeling approach of Madhusudhan & Seager (2009). The green model assumes within the 1σ uncertainties. a H-rich solar composition atmosphere in thermochemical equilibrium. The (A color version of this figure is available in the online journal.) green, red, and brown models are all H-rich but with different amounts of C and O. Whereas the green model has a solar composition of C and O in chemical equilibrium, the red model is depleted in C and O by a factor of 20 relative to solar abundances, and the brown model has no C- and O-based molecules. The atmosphere, e.g., H2O-rich, is also not favored by the data be- blue model is a water-world atmosphere with 100% H2O. The colored solid cause such a model predicts significantly lower transit depth in circles are the models binned in the bandpasses. As discussed in the text, the the visible MOST bandpass than observed, by 2σ . We found data are inconsistent (at the 2σ level) with both a solar composition atmosphere that a cloud-free H2-rich atmosphere with subsolar C and O as well as the H2O-rich scenario. A cloud-free H-rich atmosphere depleted in C abundances, by a factor of 20 below solar or lower, is able to and O can explain the data marginally, at the ∼1σ uncertainties. match both of the data points at the ∼1σ uncertainties. The low (A color version of this figure is available in the online journal.) C and O abundances in such a model lower the H2O, CO, and CO2 absorption in order to match the low 4.5 μm transit depth, The MOST bandpass is diagnostic of scattering phenomena whereas the ambient H2-rich composition contributes Rayleigh as well as absorption due to a variety of chemical species, scattering in the visible wavelengths, which together with Na including Na, K, and particulates. In contrast, the Spitzer and K absorption matches the high transit depth in the MOST 4.5 μm bandpass constrains molecular absorption, particularly bandpass. Whereas this model fits the data reasonably well, due to H2O, CO, and CO2. We used model transmission the peculiar composition with low C and O abundances poses spectra of HD 97658 b with a wide range of compositions interesting theoretical questions for future investigation. to interpret the observations. We considered two classes of The observations, again if interpreted in the context of atmo- model atmospheres: (1) cloud-free atmospheres with varied gas spheric modeling, are more readily explained by an atmosphere compositions (Madhusudhan & Seager 2009), and (2) atmo- with significant Mie scattering due to hazes. We considered at- spheres with clouds or hazes of varied particulate composi- mospheres with a wide range of haze compositions and particu- tions (Howe & Burrows 2012; see also Burrows & Sharp 1999; late sizes based on the models of Howe & Burrows (2012). We Burrows et al. 2001; Sharp & Burrows 2007). These two sets of found that the observations can be explained by the presence of model atmospheres are known to predict slightly different am- tholin hazes in a solar-abundance H2-rich atmosphere, as shown plitude variations in the 3–5 μm range for unidentified reasons in Figure 7. The models include monodispersed tholin hazes (see Howe & Burrows 2012 for a discussion about this), but this placed uniformly in the upper atmosphere, in the 10−4–10−6 does not qualitatively impact our conclusions. bar pressure range, in an otherwise solar-abundance atmosphere. The observations, if interpreted in the context of atmospheric The models shown in Figure 7 range in particle sizes between modeling, can nominally be explained by a cloud-free atmo- 0.01 and 0.1 μm and particle densities of 102–106 cm−3.All sphere but only with a metal-poor composition. Figure 6 shows of the models show a steep rise in the transmission spectrum model atmospheric spectra of HD 97658 b with varied com- in the visible wavelengths due to Mie scattering due to the positions, based on the modeling approach of Madhusudhan & hazes, thereby explaining the high observed transit depth in the Seager (2009). We considered models with H2-rich atmospheres 0.525 μm MOST bandpass. Of the models shown in Figure 7, as well as models with high-MMW, e.g., H2O-rich atmospheres. the best-fitting model contains 0.1 μm particles with a number − We found that a cloud-free solar composition H2-rich atmo- density of 100 cm 3, but it is only a marginally better fit relative sphere in the planet is inconsistent with the data (green curve to the two other models. on Figure 6, which is entirely overlapped by the red, H2-rich To conclude this discussion, we stress that future observations low-metallicity model in the optical wavelength range). Such an at higher resolution and higher precision are definitely needed atmosphere predicts a significantly higher 4.5 μm transit depth to confirm the atmospheric modeling interpretation as presented than observed, by over 2σ, due to molecular absorption by H2O, here and to further constrain the atmospheric composition of CO, and CO2 in that bandpass. In fact, a cloud-free high-MMW HD 97658 b.

8 The Astrophysical Journal, 786:2 (11pp), 2014 May 1 Van Grootel et al.

Table 3 16 M . M − Mass, Age, Effective Temperature, Luminosity, Metallicity, Large Separations, ∗ =073 ,Fe/H= 0.26 1.0Gyr and Small Separations of 12 Stellar Models Consistent with the Observational M∗ =0.77M ,Fe/H=−0.23 Constraints (Teff ,L∗,[Fe/H]) M∗ =0.81M ,Fe/H=−0.20 Δ Model M∗ Age Teff L∗ [Fe/H] νδν02 14 (M) (Gyr) (K) (L)(μHz) (μHz) 4.1Gyr 1 0.73 11.0 5144 0.337 −0.26 188.5 9.5 2 0.73 11.9 5165 0.348 −0.26 185.8 8.8 3 0.73 12.8 5187 0.362 −0.26 182.9 8.1 12 7.2Gyr

4 0.73 13.7 5210 0.377 −0.26 179.7 7.3 Hz) μ

5 0.77 7.2 5130 0.338 −0.23 191.4 11.8 (

6 0.77 8.0 5144 0.346 −0.23 189.0 11.2 02

− δν 7 0.77 9.0 5170 0.361 0.23 185.8 10.3 10 . 11.0Gyr 8 0.77 10.0 5195 0.377 −0.23 182.4 9.5 10 0Gyr 9 0.81 1.1 5152 0.337 −0.20 199.6 15.4 10 0.81 2.0 5171 0.346 −0.20 196.6 14.7 − 11 0.81 2.9 5192 0.358 0.20 193.8 14.0 8 12 0.81 4.1 5220 0.375 −0.20 190.4 13.2 13.7Gyr

6.2. Improving the Knowledge of the Host Star 6 170 180 190 200 An accurate knowledge of the host star is essential to deriving accurate planet parameters, as illustrated here by the importance Δν (μHz) of modeling properly the stellar luminosity and radius. Age Figure 8. C − D diagram (large separations Δν vs small separations δν02)for is also a very important datum because it is expected to be stellar models with various masses and metallicities but that are consistent with equal to the age of the planet. For low-mass stars such as the observational constraints (Teff ,L∗,[Fe/H]) on HD 97658. HD 97658, the stellar age is essentially unconstrained by the (A color version of this figure is available in the online journal.)  comparison with evolutionary tracks. Based on log RHK and P calibrations, the stellar age is around 6.0 ± 1.0 Gyr (Henry rot metallicities, and ages, and all simultaneously respect the et al. 2011). However, the physics behind these two empirical observational triplet (T ,L∗,[Fe/H]) within the associated methods is not fully understood, and their calibrations may eff 1σ uncertainties. Their properties are presented in Table 3. also present large unknown systematics that are not reflected We plotted in Figure 8 the Δν–δν diagram, called the C−D in the 1.0 Gyr error announced (Soderblom 2010). The UVW 02 diagram in the asteroseismic jargon (Christensen-Dalsgaard space velocities, as measured from Hipparcos (van Leeuwen 1984), for these 12 stellar models.15 Figure 8 shows that 2007), are small. This indicates that HD 97658 has thin disk an accuracy of ∼1 μHz on the large and small separations kinematics, from which no useful constraints on its age can measurements will help to get improved knowledge of the be obtained. Asteroseismology, the study of the oscillations of stellar mass and age. The main unknown for asteroseismic stars, can improve this situation (for the general principles of observations concerns the amplitudes of the oscillations, which asteroseismology and some of the most recent achievements, are not predictable by the current linear theory of stellar see Aerts et al. 2010). In solar-type (roughly mid-F to mid- oscillations. The amplitudes are expected to be weak in K-type K types) stars, oscillations correspond to acoustic waves (also stars (Kjeldsen & Bedding 1995), but HD 97658 is a quite bright called p-modes) that depend on the radially varying density and star. We estimated, from the measured CoRoT performances on internal speed of sound in the star. The power pulsation spectra similar type and magnitude stars (see the example of the K0 of solar-type stars exhibit characteristic structures with regular dwarf V = 7.84 HD 46375; Gaulme et al. 2010) that a space- spacings between the peaks. Among them, two are of utmost borne 0.3 m mission like Characterizing Exoplanet Satellite importance: the mean large separation Δν (≡ ν − ν , n+1,l n,l (CHEOPS; Broeg et al. 2013) would be able to obtain such a where ν is the frequency, n the radial order, and l the angular precision from ∼2–3 months of nearly continuous observations. degree of the oscillation mode) and the small separations δν This may be a suggestion for the complementary CHEOPS (≡ ν − ν − ). These quantities are the first quantities n,l n 1,l+2 open program. The PLATO mission, if HD 97658 falls in the that can be measured in an observed pulsation spectrum, even observed fields, will definitely be able to measure the large and if the quality of the data is insufficient to extract individual small separations with the required accuracy. The PLATO should p-mode frequencies (Roxburgh & Vorontsov 2006; Roxburgh even be able to accurately measure the individual oscillation 2009; Mosser & Appourchaux 2009). It can be shown that the frequencies, from which a full asteroseismic analysis can be large separation is approximately the inverse acoustic diameter carried out, obtaining not only very accurate global parameters of the star, which means, from homology arguments, that it (stellar mass, radius, age) but also constraints on the internal scales approximately as the square root of the mean density √ physics of the star. This is in turn important to calibrate and of the star: Δν ∝ ρ∗. For their part, the small separations δν improve the evolutionary tracks computed from stellar evolution provide a measure of conditions in the core of the star and hence codes. the stellar age. To illustrate the potential of asteroseismology to improve the knowledge of the host star, in particular by constraining its age, we computed the seismic properties of 12 selected 15 Here Δν = νn+1,0 − νn,0 and δν02 = νn,0 − νn−1,2. They are given for the stellar models with the Liege adiabatic oscillation code OSC corresponding frequency at maximum power, around which the pulsation (Scuflaire et al. 2008a). These models have various masses, spectrum is expected to be observed.

9 The Astrophysical Journal, 786:2 (11pp), 2014 May 1 Van Grootel et al. 7. CONCLUSION AND PROSPECTS Support for this work was provided by NASA. M. Gillon is Research Associate at the Belgian Fonds de la Recherche HD 97658 b is a key transiting super-Earth. The current data Scientifique (FNRS). from Spitzer and MOST photometry, and also Keck–HIRES Facilities: Keck:I (HIRES), MOST, Spitzer RVs, analyzed with our MCMC code, indicate a planet with = +0.70 −3 an intermediate average density (ρP 3.90−0.61 gcm ). REFERENCES Investigating the possible internal compositions of HD 97658 b, our results indicate a large rocky component of at least 60% by Absil, O., Milli, J., Mawet, D., et al. 2013, A&A, 559, L12 mass, an amount of 0%–40% of water+ices, and very little H–He Aerts, C., Christensen-Dalsgaard, J., & Kurtz, D. W. 2010, Asteroseismology (Berlin: Springer) components, at most 2% by mass. If interpreted as constraints Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481 on the atmospheric composition, the transit depths in both the Baglin, A., Auvergne, M., Barge, P., et al. 2006, in The CoRoT Mission Pre- Spitzer and MOST bandpasses together favor either an H2-rich Launch Status—Stellar Seismology and Planet Finding, ed. M. Fridlund, A. atmosphere with hazes or a cloud-free atmosphere with a low Baglin, J. Lochard, & L. Conroy (ESA SP-1306; Noordwijk: ESA), 33 metallicity. Six more transit observations are planned in the Bean, J. L., Miller-Ricci Kempton, E., & Homeier, D. 2010, Natur, 468, 669 Benneke, B., & Seager, S. 2013, ApJ, 778, 153 course of 2014 with the Spitzer IRAC camera, this time with Berta, Z. K., Charbonneau, D., Bean, J., et al. 2011, ApJ, 736, 12 3.6 μm channel (PI: D. Dragomir). Berta, Z. K., Charbonneau, D., Desert,´ J.-M., et al. 2012, ApJ, 747, 35 HD 97658 b will be a target of the coming space mis- Bohm-Vitense,¨ E. 1958, ZA, 46, 108 sions TESS and CHEOPS, in particular to accurately measure Boyajian, T. S., von Braun, K., van Belle, G., et al. 2013, ApJ, 771, 40 the planet radius at visible wavelengths. By the time TESS Broeg, C., Fortier, A., Ehrenreich, D., et al. 2013, in European Physical Journal 16 Web of Conferences, Hot Planets and Cool Stars, ed. R. Saglia (Les Ulis, and CHEOPS are launched (circa 2017), the Gaia mission France: EDP Sciences), 47, 3005 (launched in 2013 December) is hoped to have provided ex- Burrows, A., Hubbard, W. B., Lunine, J. I., & Liebert, J. 2001, RvMP, 73, 719 tremely accurate parallax measurements, within ∼10 μas for Burrows, A., & Sharp, C. M. 1999, ApJ, 512, 843 the bright nearby stars such as HD 97658. This will improve the Carlin, B. P., & Louis, T. 2008, Bayesian Methods for Data Analysis (3rd ed.; London: Chapman and Hall) knowledge on the distance of the star by two orders of magni- Christensen-Dalsgaard, J. 1984, in Space Research in Stellar Activity and tude compared to the current Hipparcos parallax. From there, Variability, ed. A. Mangeney & F. Praderie (Meudon, France: Observatoire the stellar radius will be known with an accuracy of 1% (assum- de Paris), 11 ing a ± 50 K uncertainty on Teff). Knowing the stellar radius to Christensen-Dalsgaard, J., & Daeppen, W. 1992, A&ARv, 4, 267 1% is also what we can expect for present and near-future long Claret, A., & Bloemen, S. 2011, A&A, 529, A75 de Mooij, E. J. W., Brogi, M., de Kok, R. J., et al. 2012, A&A, 538, A46 baseline interferometers (Boyajian et al. 2013). This is the ulti- Demory, B.-O., Gillon, M., Deming, D., et al. 2011, A&A, 533, A114 mate precision we can achieve on the planet radius, providing Demory, B.-O., Gillon, M., Seager, S., et al. 2012, ApJL, 751, L28 that the number of observed transits is sufficient to measure the Desert,´ J.-M., Bean, J., Miller-Ricci Kempton, E., et al. 2011, ApJL, 2 transit depth dF ∝ (RP /R∗) with an accuracy much below 1%. 731, L40 The ESA has now officially confirmed the selection of PLATO as Dragomir, D., Matthews, J. M., Eastman, J. D., et al. 2013, ApJL, 772, L2 Dragomir, D., Matthews, J. M., Howard, A. W., et al. 2012, ApJL, 759, L41 the next M3 mission, to be launched around 2024. If HD 97658 Escobar, M. E., Theado,´ S., Vauclair, S., et al. 2012, A&A, 543, A96 falls in the observed fields, PLATO would not only measure the Fazio, G. G., Hora, J. L., Allen, L. E., et al. 2004, ApJS, 154, 10 transit depth with unprecedented accuracy, but would also be Ferguson, J. W., Alexander, D. R., Allard, F., et al. 2005, ApJ, 623, 585 able to detect oscillation frequencies of the host star that pro- Flower, P. J. 1996, ApJ, 469, 355 Fressin, F., Torres, G., Charbonneau, D., et al. 2013, ApJ, 766, 81 vide through asteroseismology accurate stellar mass and, very Gaulme, P., Deheuvels, S., Weiss, W. W., et al. 2010, A&A, 524, 47 importantly, age. Gelman, A., & Rubin, D. 1992, StaSc, 7, 457 Orbiting a bright K1-type star (V = 7.7, K = 5.7), Gilliland, R. L., Marcy, G. W., Rowe, J. F., et al. 2013, ApJ, 766, 40 HD 97658 b is a target of choice for atmospheric characteriza- Gillon, M., Deming, D., Demory, B.-O., et al. 2010, A&A, 518, A25 tion. There is Hubble Space Telescope time allocated in the com- Gillon, M., Demory, B.-O., Benneke, B., et al. 2012a, A&A, 539, A28 Gillon, M., Demory, B.-O., Deming, D., Seager, S., & Lovis, C. 2011, in IAU ing months to take transmission spectroscopy of HD 97658 b Symp. 276, The Astrophysics of Planetary Systems: Formation, Structure, with the WFC3 camera in IR (PI: H. Knutson). The aim is to and Dynamical Evolution, ed. A. Sozzetti, M. G. Lattanzi, & A. P. Boss distinguish a large scale-height, H-dominated atmosphere from (Cambridge: Cambridge Univ. Press), 167 a compact, water–steam atmosphere, which will confirm or re- Gillon, M., Pont, F., Moutou, C., et al. 2006, A&A, 459, 249 fute the constraints on the atmospheric composition derived in Gillon, M., Triaud, A. H. M. J., Fortney, J. J., et al. 2012b, A&A, 542, A4 Henry, G. W., Howard, A. W., Marcy, G. W., Fischer, D. A., & Johnson, J. A. this work. By comparing these measurements to the few targets 2011, arXiv:1109.2549 for which such transmission spectroscopy is currently possible Holman, M. J., Winn, J. N., Latham, D. W., et al. 2006, ApJ, 652, 1715 (55 Cnc e and GJ 1214 b), this will provide the first measure Howard, A. W. 2013, Sci, 340, 572 of the diversity of super-Earth atmospheres prior to the era of Howard, A. W., Johnson, J. A., Marcy, G. W., et al. 2011, ApJ, 730, 10 Howe, A. R., & Burrows, A. S. 2012, ApJ, 756, 176 the European Extremely Large Telescope and the James Webb Iglesias, C. A., & Rogers, F. J. 1996, ApJ, 464, 943 Space Telescope. Izotov, Y. I., & Thuan, T. X. 2010, ApJL, 710, L67 Kjeldsen, H., & Bedding, T. R. 1995, A&A, 293, 87 The authors thank the referee for constructive comments Knutson, H. A., Charbonneau, D., Allen, L. E., Burrows, A., & Megeath, S. T. that improved the quality of the manuscript. V. Van Grootel 2008, ApJ, 673, 526 Koch, D. G., Borucki, W. J., Basri, G., et al. 2010, ApJL, 713, L79 warmly thanks A. Grotsch-Noels and M.-A. Dupret for fruitful Koen, C., Kilkenny, D., van Wyk, F., & Marang, F. 2010, MNRAS, discussions on stellar modeling. The authors also thank H. 403, 1949 Knutson for various discussions on this planet. This work is Kreidberg,L.,Bean,J.L.,Desert,´ J.-M., et al. 2014, Natur, 505, 69 based in part on observations made with the Spitzer Space Kuzuhara, M., Tamura, M., Kudo, T., et al. 2013, ApJ, 774, 11 Telescope Madhusudhan, N., & Seager, S. 2009, ApJ, 707, 24 , which is operated by the Jet Propulsion Laboratory, Mandel, K., & Agol, E. 2002, ApJL, 580, L171 California Institute of Technology, under a contract with NASA. Marois, C., Zuckerman, B., Konopacky, Q. M., Macintosh, B., & Barman, T. 2010, Natur, 468, 1080 16 http://sci.esa.int/gaia Miller-Ricci, E., Seager, S., & Sasselov, D. 2009, ApJ, 690, 1056

10 The Astrophysical Journal, 786:2 (11pp), 2014 May 1 Van Grootel et al.

Miller-Ricci Kempton, E., Zahnle, K., & Fortney, J. J. 2012, ApJ, 745, 3 Seager, S., Kuchner, M., Hier-Majumder, C. A., & Militzer, B. 2007, ApJ, Morley, C. V., Fortney, J. J., Kempton, E. M.-R., et al. 2013, ApJ, 775, 33 669, 1279 Mosser, B., & Appourchaux, T. 2009, A&A, 508, 877 Sharp, C. M., & Burrows, A. 2007, ApJS, 168, 140 Petigura, E. A., Marcy, G. W., & Howard, A. W. 2013, ApJ, 770, 69 Soderblom, D. R. 2010, ARA&A, 48, 581 Rauer, H., Catala, C., Aerts, C., et al. 2014, Exp. Astron., in press Stetson, P. B. 1987, PASP, 99, 191 (arXiv:1310.0696) Torres, G. 2010, AJ, 140, 1158 Ricker, G. R., Latham, D. W., Vanderspek, R. K., et al. 2010, BAAS, 42, 215 Valencia, D., Guillot, T., Parmentier, V., & Freedman, R. S. 2013, ApJ, Roxburgh, I. W. 2009, A&A, 506, 435 775, 10 Roxburgh, I. W., & Vorontsov, S. V. 2006, MNRAS, 369, 1491 Valencia, D., O’Connell, R. J., & Sasselov, D. 2006, Icar, 181, 545 Schwarz, G. 1978, AnSta, 6, 461 Valencia, D., Sasselov, D. D., & O’Connell, R. J. 2007, ApJ, 665, 1413 Scuflaire, R., Montalban,´ J., Theado,´ S., et al. 2008a, Ap&SS, 316, 149 van Leeuwen, F. 2007, A&A, 474, 653 Scuflaire, R., Theado,´ S., Montalban,´ J., et al. 2008b, Ap&SS, 316, 83 Winn, J. N., Matthews, J. M., Dawson, R. I., et al. 2011, ApJL, 737, L18

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